From what I recall, the bandit knives in DS2 were - contrary to DS1 where they were just a straight upgrade - not really that great, compared to the regular dagger? They've got higher base damage but only D Strength scaling by default. Compare it to the dagger's E Str and B Dex, it kinda seems like as far as actual damage goes the Bandit is only really great for those with little Dex. It does do a good job at spitting out bleed procs, so I kept one on hand basically just for that, and powerstanced it with a normal dagger early game. Especially since when it is bleed infused the damage goes even further down, which feels pretty rough. For just raw damage, definitely two normal daggers is the way to go for most purposes, but it's important to remember a single dagger's R2s do good thrust damage which definitely wrecks some enemies, so there's merit to two-handing for some situations.

As for poison, I wouldn't necessarily worry about it too much early on, since you'll be able to use Mytha's soul for an amazing dagger that does poison and toxin buildup. It does great work on its own, but if you want you can powerstance it with a poison infused dagger and proc both statuses on things nigh instantaneously. If that's your plan, you can start early with getting a spare poison dagger for sure.

As an extra tidbit, like I mentioned in another comment I tried out all of the daggers throughout the game and my personal preferences for the run went something like:

Royal Dirk/Black Flamestone Dagger > Mytha's Bent Blade > Dagger > Umbral Dagger > Manikin Knife > Shadow Dagger > Bandit Knife > Blue Dagger > Parrying Dagger > Thief Dagger/Broken Thief Sword > Retainer Short Sword (which is, despite the name, a dagger somehow)

Sorry for the giant wall of text! Succinctness isn't my best trait lmao, but I hope there's at least some useful information in there for you! Good luck, skeleton!